1. Field of the Invention
The present invention pertains to remote sensing, and, more particularly, to the employment of a multipath mitigation filter in such applications.
2. Description of the Related Art
FIG. 1 depicts a classic example what is known as a xe2x80x9cmultipathxe2x80x9d condition giving rise to a xe2x80x9cmultipathxe2x80x9d error such as is addressed by the present invention. xe2x80x9cMultipathxe2x80x9d describes the condition where more than one path exists for the propagation of signals, such as electromagnetic or acoustic signals. Consider the generic radar scenario illustrated in FIG. 1. A radar illuminates an object and measures the signal energy, which returns by four different paths. A portion of the energy is:
directly transmitted to the object and returns from the object along the path Rxe2x86x92Oxe2x86x92R;
directly transmitted to the object and returns indirectly by bouncing off the ground along the path Rxe2x86x92Oxe2x86x92R;
indirectly transmitted to the object by bouncing of the ground and returns directly to the object along the path Rxe2x86x92Gxe2x86x92Oxe2x86x92R; and
indirectly transmitted to the object by bouncing off the ground and returns indirectly by bouncing off the ground along the path Rxe2x86x92Gxe2x86x92Oxe2x86x92Gxe2x86x92R.
Each path has a different time-delay and different signal attenuation. Under the proper conditions, the direct signal and the indirect signal(s) interact. Exemplary factors defining xe2x80x9cproper conditionsxe2x80x9d include primarily object altitude, radar height, object-to-radar range, radar wavelength, radar beam width, and terrain type, although there are others. This phenomenon is also encountered in applications employing other kinds of energy, e.g., remote sensing applications employing sonar, synthetic aperture radar, sonograms, CAT scans, PET scans, etc.
The interaction can result in reinforcement or cancellation of the returned signals. In either case, the multipath conditions introduce error. This is sometimes referred to as xe2x80x9cinterferencexe2x80x9d since the reflected signals interfere with one another, but is also sometimes referred to as xe2x80x9cdistortion.xe2x80x9d If the returned signals are in phase, the interference is constructive. If out of phase, the interference is destructive and degrades the quality of the radar""s measurements significantly. In particular, monopulse tracking radars suffer severe multipath induced elevation measurement errors. And, when the conditions are just right, the received radar signals can completely cancel each other.
For a ground-based radar, multipath is encountered tracking low-altitude objects. The primary manifestations are low signal-to-noise ratio at the radar""s receiver, very noisy measurements, and large elevation errors. More particularly, a track from a radar experiencing multipath distortion may be biased up or down and be noisy. The multipath problem is worse for a sea-based radar because salt water, being electrically conductive, reflects radar signals like a mirror. The direct and indirect returns can destructively interfere such that a very low power signal is received by the radar during an extended period of time. Consequently, the measurement noises increase during the presence of multipath and in some cases, the radar may even lose track because the object xe2x80x9cfades away.xe2x80x9d In other words, the returned signal level drops below the receivers"" detection level.
Multipath detection can be accomplished in monopulse radars by processing the monopulse elevation quadrature signal. However, in many applications, monopulse quadrature signals are not available, and an alternate multipath detection scheme is needed.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.
The invention is a technique for estimating the position of an object from return signals in the presence of multipath conditions. In its various aspects, the invention is manifested as a position estimator; a method for estimating the position of a object from returned signals; a program storage medium encoded with instructions that, when executed, perform such a method; and a computing device programmed to perform such a method. The position estimator, in one embodiment, comprises a signal filter, a multipath detector, and a multipath bias estimator. The signal filter is capable of receiving a sensor measurement and generating a measurement noise variance estimate and a position estimate therefrom. The multipath detector is capable of applying a dynamic threshold against the measurement noise variance estimate to determine whether a multipath condition exists. And, the multipath bias estimator is capable of generating a correction for multipath induced measurement bias error from a measurement noise variance estimate for application upon determining that a multipath condition exists. The method includes filtering the sensor measurement with a measurement noise variance estimator to yield a measurement noise variance estimate; comparing the estimate against a dynamic threshold to detect whether a multipath condition exists; determining a correction for a multipath induced measurement bias error from the estimate if the multipath condition is detected; and applying the correction to the estimation of the object""s position.